Metastasis is the transfer of malignant tumors from one organ to a distant organ. It is the most common cause of death in cancer patients. Different molecular mechanisms enable tumor cells to infiltrate the surrounding tissue, invade blood vessels and leave the blood stream at a different site. Tumor cell interaction with extracellular matrix components and epithelial-mesenchymal transition as well as angiogenesis are important factors in invasion and metastasis. Gene expression profiles of metastatic cells in various organs are different and not every tumor cell has the capacity to metastasize. The microenvironment in the organ influences formation of metastasis. Only 1% of micrometastases progress into macrometastases.